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Fluid Phase Equilibria 299 (2010) 285–293

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Fluid Phase Equilibria
journal homepage: www.elsevier.com/locate/fluid

SRK equation of state: Predicting binary interaction parameters of hydrocarbons and related compounds
Giorgio Soave a,∗ , Simone Gamba b , Laura A. Pellegrini b
a b

Via Europa 7, I-20097 San Donato Milanese, Italy Dipartimento diChimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy

a r t i c l e

i n f o

a b s t r a c t
The parameter mixing rules of the Soave–Redlich–Kwong (SRK) equation of state are rewritten as Huron–Vidal mixing rules, where infinite-pressure activity coefficients are predicted by group contributions. Alkanes are treated ascomposed by one group type and aromatics by two types, aliphatic and aromatic. Hydrocarbon mixtures can be treated using one universal interaction parameter. Light compounds like methane, N2 , CO2 , H2 S, etc. are treated as separate groups; each one requires a pair of parameters for its interactions with aliphatic and aromatic groups. Group interaction parameters were determined fromexperimental VLE data. From them, binary interaction constants of the classical quadratic mixing rules can directly be derived. © 2010 Elsevier B.V. All rights reserved.

Article history: Received 25 May 2009 Received in revised form 6 September 2010 Accepted 7 September 2010 Available online 17 September 2010 Keywords: Equation of state Cubic Mixing rules Group contributions

1. Introduction A widelyapplied equation of Soave–Redlich–Kwong (SRK) one [1]: P= RT state (EoS) is the

v−b

−

a

v(v + b)

(1)

The so-called ‘classical’ parameter mixing rules: a=
i j

xi xj aij =
i j

xi xj (1 − kij )

ai aj

(2)

b=
i

xi bi

(3)

can give a satisfactory description of the vapor–liquid-equilibrium (VLE) behaviour of systems formed by hydrocarbons and related compoundslike nitrogen, carbon dioxide and hydrogen sulfide. For its simplicity it is widely applied for process design in the hydrocarbon field. Binary parameters kij are not known in general, however. Relatively few values have been determined by correlation of experimental VLE data and are reported in the literature (e.g., [2]). Zero values of kij can be, and usually are, applied for alkane–alkane pairs,but for others (especially those including non-hydrocarbons)

they should be different from zero and they have to be determined; that requires to retrieve and correlate experimental VLE data of each pair of compounds, a practically impossible task. For that reason methods have been developed in the last decades, which determine the parameters of the Redlich–Kwong or the Peng–Robinson EoSs fromactivity coefficients predicted by the UNIFAC model which had to be extended to light compounds (hydrogen, nitrogen, carbon oxides, etc.). Two methods were made available by Gmehling and co-workers: the widely applied PSRK method [3] and the more recent and still incomplete VTPR method [4] based on the Peng–Robinson EoS. In principle, they enable to predict the phase equilibrium and otherthermodynamic properties of a wide variety of systems. This work was mainly aimed to develop a tool to predict EoS parameters from a small number of universal group interaction parameters. The new method is restricted to hydrocarbons and related components (N2 , CO2 , H2 S), i.e., systems where the classical quadratic mixing rules apply. It can be used to directly estimate parameters of the SRK EoS, or thebinary interaction parameters kij for the classical mixing rules.

2. The Huron–Vidal mixing rules Many years ago Huron and Vidal [5] proposed a different type of mixing rules for the SRK EoS. They assumed that the excess properties of mixtures have a finite limit at infinite pressure. The rules they derived from that, gave rise to the mixing rules expressed by

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